Optical Unit

ABSTRACT

An optical unit includes a light source that emits light to be projected onto a screen, one or a plurality of optical elements that controls a spread of a beam of light from the light source to the screen, a combining element that combines the light emitted from the light source, and a scanning element. The light source outputs an optical beam that will generate an elliptically shaped beam spot on the screen, the beam spot having a major axis substantially perpendicular to a scanning direction, and the light source is driven with pulse width modulation (PWM) so that one driven pulse corresponds to one beam spot on the screen.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/672,744, filed Nov. 9, 2012 which is a continuation of Ser. No.12/759,027, filed Apr. 13, 2010, now U.S. Pat. No. 8,308,301, thecontents of which are incorporated herein by reference.

This application claims priority from Japanese Patent Application JPP2009-097589, filed on Apr. 14, 2009, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to an image display device that displaysan image by scanning a screen with an optical beam of a light sourceafter modulation of the beam according to a particular level of an imagesignal. More particularly, the invention relates to miniaturizing anoptical unit.

The new types of displays such as a DMD (Digital Mirror Device) type,liquid-crystal panel type, or optical scanning type display, whichemploy lasers or LEDs as light sources, are coming into existence toconstruct a large screen with a compact image-display device design.Using a white light source, for example, allows the volume of the lightsource to be reduced remarkably and thus the image display device to bedimensionally reduced. Using a monochromatic light source of, forexample, red (R), green (G), and blue (B), further allows finer-imageformation in addition to miniaturization.

JP-A-2007-293226, for example, discloses a laser display device thatcomprises focusing means including a plurality of laser diode elementseach emitting laser light and scanning means for reflecting the laserlight so that an image is projected onto a screen.

SUMMARY OF THE INVENTION

The conventional device that uses the technique disclosed inJP-A-2007-293226, however, is expensive since the plurality of laserlight sources are arranged proximately. The conventional device hasanother problem in that since a large amount of heat occurs, opticalaxes of the light sources become misaligned and/or optical outputintensity decreases. The device further presents the problem ofdegradation in resolution due to variations in beam spot sizes of themultiple lasers.

Accordingly, an object of the present invention is to provide: anoptical unit that is simpler in structure, less expensive, and capableof offering higher-resolution image quality while reducing devicedimensions and weight; and an image display device using the opticalunit.

In order to attain the above object, an optical unit according to anaspect of the present invention comprises one or a plurality of lightsources, one or a plurality of optical elements each controlling aspread of light, a combining element that combines optical beams emittedfrom each light source, and a scanning element; wherein the light sourceoutputs an optical beam that will generate an elliptically shaped beamspot on the screen, the beam spot having a major axis substantiallyperpendicular to a scanning direction.

The present invention makes achievable a compact optical unit capable ofproviding high-resolution image quality, and an image display deviceusing the optical unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically shows a configuration of an optical unit accordingto a first embodiment of the present invention;

FIG. 1B shows a beam spot formed on a screen during projection by animage display device using the optical unit of the present invention;

FIG. 1C shows beam spots formed on a screen, and luminous intensitylevels obtained, during projection using a conventional driving method;

FIG. 1D shows beam spots formed on the screen, and luminous intensitylevels obtained, during projection by the image display device using theoptical unit of the present invention;

FIG. 2 schematically shows a configuration of an optical unit accordingto a second embodiment of the present invention;

FIG. 3 schematically shows a configuration of an optical unit accordingto a third embodiment of the present invention;

FIG. 4 schematically shows a configuration of an optical unit accordingto a fourth embodiment of the present invention;

FIG. 5 schematically shows a configuration of an optical unit accordingto a fifth embodiment of the present invention;

FIG. 6 schematically shows a configuration of an optical unit accordingto a sixth embodiment of the present invention;

FIG. 7A is a diagram that shows a light source and optical element ofthe optical unit of the present invention, and a screen;

FIG. 7B is a diagram that shows the screen onto which an image isprojected by the image display device using the optical unit of thepresent invention, and pixels in a scanning direction on the screen;

FIG. 7C is a diagram that shows one pixel on the screen and one beamspot formed thereon during projection by the image display device usingthe optical unit of the present invention;

FIG. 8 is another diagram that shows one pixel on the screen and onebeam spot formed thereon during projection by the image display deviceusing the optical unit of the present invention;

FIG. 9 is yet another diagram that shows a white beam formed on thescreen during projection using the optical unit of the presentinvention;

FIG. 10A is a further diagram that shows beam spots formed on the screenduring projection by the image display device using the optical unit ofthe present invention;

FIG. 10B is a further diagram that shows beam spots formed on the screenduring projection by the image display device using the optical unit ofthe present invention;

FIG. 11 schematically shows a configuration of an optical unit accordingto a seventh embodiment of the present invention;

FIG. 12 schematically shows a configuration of an optical unit accordingto an eighth embodiment of the present invention;

FIG. 13A is a diagram that schematically shows an example oftime-varying changes in optical output level that are observed when theoptical unit of the present invention is driven;

FIG. 13B is a diagram showing an example of a conventional drivingmethod (PWM driving);

FIG. 13C is a diagram showing another example of a conventional drivingmethod (analog driving);

FIG. 14 schematically shows a configuration of an optical unit accordingto a ninth embodiment of the present invention; and

FIG. 15 is a diagram that shows an embodiment of an image displaydevice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below with referenceto the accompanying drawings.

FIG. 1A schematically shows a configuration of an optical unit accordingto a first embodiment of the present invention. The optical unit 12according to the present embodiment includes a plurality of lightsources 1 a, 1 b, and 1 c, a plurality of optical elements 2 a, 2 b, and2 c that each control a spread of light, an element 3 that combines thelights emitted from the light sources, and a scanning element 4. Thelight from each of the light sources 1 a, 1 b, and 1 c generates anelliptically shaped spot, a major axis of which is nearly perpendicularto a scanning direction as shown in FIG. 1B. Thus, for example, in acase where a plurality of isochromatic light sources are used as thelight sources 1 a, 1 b, and 1 c, even if a temperature change causespositions of light-emitting points of the light sources 1 a, 1 b, and 1c to shift or even if mounting positions themselves of the light sources1 a, 1 b, and 1 c shift, high luminance can be achieved withoutconspicuous misalignment of respective optical axes occurring.Conversely in a case where monochromatic light sources different fromone another are used as the light sources 1 a, 1 b, and 1 c, a colorcombiner 3 such as a color-combining mirror, prism, light guide, oroptical fiber, will be used as the combining element 3. Timing ofactivation of the light sources can be controlled in the scanningdirection, so a shift in color can be prevented from occurring, but thecontrol is difficult in a direction perpendicular to the scanningdirection. Meanwhile, the present invention has an advantage in thatsince spots can be disposed to be elongate with respect to the scanningdirection, even if spots of different colors shift in position,overlapping area sizes of the spots will be large enough to preventoptical-axis misalignment and color shifting.

FIG. 1C shows beam spots formed on a screen, and luminous intensitylevels obtained, during projection using a conventional driving method.In the conventional method, the fact that sizes of the beam spots forone pixel are nearly equal to one another and adjacent pixels are toodark has blurred the image, making high-resolution sharp image displaydifficult.

In the present invention, however, images are displayed at highresolution since, as shown in FIG. 1D, minor axes of the spots are shortfor one pixel, and thus since the light sources 1 a, 1 b, and 1 c can bedeactivated before the spots enter a next pixel.

For example, if the light sources 1 a, 1 b, and 1 c here are of a lasertype, electric power can be saved by stopping the laser oscillationsduring a black-color display period, since laser light can be modulatedaccording to a particular image signal level. The light sources 1 a, 1b, and 1 c are secured using an ultraviolet (UV) curing resin or fixedby engagement. In order to provide highly sharp images by controlling aspread of the diffuse light emitted from each of the light sources 1 a,1 b, and 1 c, the optical elements 2 a, 2 b, and 2 c, such ascollimating and/or focusing lenses, optical fibers, or hologramelements, are disposed at immediate rear of the light sources. Shapes ofthe lenses in this case are convex, concave, spherical, ornon-spherical. Cylindrical lenses may be disposed to shape the beamsemitted from the light sources. The shaping allows incidence of thebeams appropriately shaped according to a shape of a reflecting portionformed as part of the scanning element 4, and high efficiency istherefore obtained. At the same time, increases in temperature due toprotrusion of any beams from the reflecting portion are suppressed.

The optical elements 2 a, 2 b, and 2 c may be disposed immediately atrear of structures of the light sources 1 a, 1 b, and 1 c, oralternatively the former may be disposed near the light-emitting pointsinside the structures. The disposition near the light-emitting pointsmakes the optical elements 2 a, 2 b, and 2 c acquire light moreefficiently, as well as allowing generation of smaller spots on thescreen 6 and hence, further improvement of image quality. The presentembodiment assumes spot diameters of 1 mm or less on the reflectingportion of the scanning element 4. An actuator or a feedback sensor maybe mounted such that the positions of the optical elements 2 a, 2 b, and2 c are automatically controllable according to the optical-axes of thelight-emitting points of the light sources 1 a, 1 b, and 1 c. Thisprevents the misalignment of the optical axes and provides highresolution.

The light sources 1 a, 1 b, and 1 c used can be white or each can be ofeither a three-color type including R (red), G (green), and B (blue), ora four-color type including R (red), G1 (green 1), G2 (green 2), and B(blue), or including R (red), G (green), B (blue), and Y (yellow). Whitelight sources 1 a, 1 b, and 1 c enhance luminance. Monochromatic lightsources 1 a, 1 b, and 1 c of the three-color or four-color typessignificantly improve color reproducibility, enhancing image quality. Aplurality of light sources 1 a, 1 b, and 1 c that are 2 to 5 nanometersdifferent from one another in wavelength may be used. This allowsreduction of, for example, a speckle pattern caused by interference iflaser light sources 1 a, 1 b, and 1 c are used.

In addition, if LEDs or lasers are used as the light sources 1 a, 1 b,and 1 c, and a dichroic mirror or a dichroic prism is used as thecombining element 3, it suffices just to obtain optical characteristicsof its dichroic surface in a wavelength region enabling the lightsources 1 a, 1 b, and 1 c to exhibit at least 10% of peak intensity onbeam profiles (laser light intensity distribution diagrams) of the lightsources. If the LED or laser light sources 1 a, 1 b, and 1 c have apeaked light-emission distribution, not a broad one, it suffices just toobtain a transmittance/reflectance of at least 94%, as one opticalcharacteristic of the dichroic surface, in a range of approximately+/−10 nm of a peak wavelength. For this reason, the number of layers onthe dichroic surface can be reduced and thus the optical unit can bemanufactured at a lower cost.

The plurality of light sources 1 a, 1 b, and 1 c may be mounted inindependent packages or in one package. Additionally, either thecolor-combining element 3 or the scanning element 4 may be mounted, withthe light sources 1 a, 1 b, and 1 c, in one package. Mounting inindependent packages offers advantages in that heat becomes easy torelease and in that the light sources 1 a, 1 b, and 1 c elude a peakwavelength drift and an optical loss. Meanwhile, mounting in one packageis advantageous in that using monochromatic light sources 1 a, 1 b, 1 crenders both color combination and optical axis alignment easy.

The scanning element 4 may include one two-dimensional scanning memberor two one-dimensional scanning members. The present embodiment assumesthat the reflecting portion of the scanning element 4 has either around, elliptical, square, or rectangular shape. The present embodimentalso assumes that the reflecting portion is a maximum of 1 mm in size.Making the reflecting portion have a shape that fits the shape of thebeam emitted from each of the light sources 1 a, 1 b, and 1 c, and havea minimum size, weight can be reduced and a driving speed of the opticalunit can be correspondingly increased for finer image quality.

The reflecting portion of the scanning element 4 uses, for example, analuminum- or silver-deposited or dielectric, multilayered film, as itsmaterial. This material may be top-coated with SiO₂, TiO₂, or the like.Provided that a reflectance of at least 90% is obtained in thewavelength region that enables at least 10% of the peak intensity to beachieved on the beam profiles of the light sources 1 a, 1 b, and 1 c,high-luminance images can be obtained and the amount of heat generatedby the scanning element 4 can be reduced. The optical unit 12 cantherefore be enhanced in efficiency.

The optical unit 12 can have an aperture 44, which will be disposed, forexample, at immediate rear of an exit port of any one of the lightsources 1 a, 1 b, and 1 c, optical elements, and a combining element 3,or at an exit port of a housing 444. This will make it possible toremove flare that the particular light source itself has, and to removeany interference fringes, scattered light, stray light, spot shifts, andother unfavorable factors generated by the structures of the lightsources 1 a, 1 b, and 1 c, the optical elements 2 a, 2 b, and 2 c, andthe combining element 3. Beams of either a round, elliptical, square, orrectangular shape, will then be acquirable. In a case where the aperture44 is disposed at the exit port of the housing 444, any scattered beamsthat may have impinged upon, for example, a torsion bar or the like,except at the reflecting portion of the scanning element 4, can beremoved for finer spots.

The aperture 44 has an oblique cross-section, which may look like astairway. This is effective for preventing diffracted light fromoccurring. The aperture is black in color, and is manufactured by, forexample, alumite machining of a metal such as aluminum. An elongatedhole for mounting the aperture 44 is provided in a bottom portion of thehousing 444 in order to retain the aperture accurately in the xdirection in the FIG. 1A. Accordingly, even if the light sources 1 a, 1b, and 1 c, the combining element 3, or other elements are disposedaskew, the position of the aperture 44 can be adjusted to suit theoptical axes, so that high-resolution spots with minimum flare can beobtained.

The housing 444 is formed from a metal such as aluminum, and is easy towork at a low cost. Hold members of the light sources 1 a, 1 b, and 1 c,optical elements 2 a, 2 b, and 2 c, and combining element 3, areindependent of one another, each including a heat-releasing member.Thus, even if the light sources 1 a, 1 b, and 1 c each send off a largeamount of heat, the heat can be released from the light sources 1 a, 1b, and 1 c, without damaging the optical elements 2 a, 2 b, and 2 c andthe combining element 3. After sufficient release of the heat from thelight sources 1 a, 1 b, and 1 c, higher image quality can be obtained bypreventing the occurrence of a peak wavelength drift, an optical outputloss, and shifts in the positions of the light-emitting points. Forexample, even if the optical unit 12 of the present invention is mountedin a motor vehicle or a mobile phone and an ambient temperature of theoptical unit 12 exceeds a specified range of the light sources 1 a, 1 b,and 1 c, temperatures of the light sources can be lowered to stay withintheir guaranteed operating temperature ranges, such that high luminancecan be obtained. A cushioning material may be disposed inside thehousing 444 to enhance its impact resistance.

The housing 444 has an elongate hole in a holding position of thescanning element 4. This makes the scanning element 4 adjustable in thex direction in the FIG. 1A so that beams enter the scanning element 4without loss, even if the light sources 1 a, 1 b, and 1 c and/or thecombining element 3 are disposed askew. The occurrence of heat can thusbe suppressed.

FIG. 2 schematically shows a configuration of an optical unit accordingto a second embodiment of the present invention. The combining element 3in the optical unit 12 is a color-combining element that conducts colorsyntheses upon p-polarized light.

The p-polarized light here refers to light waves whose electric fieldcomponents are parallel to a plane of incidence. In contrast,s-polarized light refers to light waves whose electric field componentsare perpendicular to the plane of incidence. For example, in a casewhere the color-combining element 3 is a dichroic prism and additionallya dichroic mirror and the light sources 1 a, 1 b, and 1 c have alight-emission distribution of a broad wavelength band, leading thep-polarized light to the color-combining element 3 enhancestransmittance and reflectance, thus enhancing luminance.

The color-combining element 3 is provided with anti-reflective (AR)coating on various faces, to prevent unnecessary reflection. Thecolor-combining element 3 is fixed to the housing 444 via an adhesiveagent. The housing 444 has a groove formed for releasing an excessadhesive at the bottom thereof, hence allowing the color-combiningelement 3 to be accurately held without a clearance from the bottom.

FIG. 3 schematically shows a configuration of an optical unit accordingto a third embodiment of the present invention. The combining element 3in the optical unit 12 is a color-combining element. The optical unit 12includes the light sources 1 a, 1 b, and 1 c that oscillate in TE(Transverse Electric) mode, the color-combining element 3 that the lightemitted from each of the light sources 1 a, 1 b, and 1 c will enter ass-polarized light, and a plane where the scanning element 4 is disposeddiffers from that where the color-combining element 3 is disposed. Inthis configuration of the optical unit 12, the major axis of a beam spoton a screen 6 can be made nearly perpendicular to a scanning direction,and high-resolution images free from conspicuous optical-axismisalignment can be obtained. The TE mode here refers to a state inwhich light waves vibrate in a minor-axis direction.

FIG. 4 schematically shows a configuration of an optical unit accordingto a fourth embodiment of the present invention. The combining element 3in the optical unit 12 is a color-combining element. The optical unit 12includes the light sources 1 a, 1 b, and 1 c that oscillate in TM(Transverse Magnetic) mode, and the color-combining element 3 that thelight emitted from each of the light sources 1 a, 1 b, and 1 c willenter as s-polarized light. In this configuration of the optical unit12, the major axis of a beam spot on a screen 6 can be made nearlyperpendicular to a scanning direction, and high-resolution images freefrom conspicuous optical-axis misalignment can be obtained. The TM modehere refers to a state in which light waves vibrate in a minor-axisdirection.

FIG. 5 schematically shows a configuration of an optical unit accordingto a fifth embodiment of the present invention. The combining element 3in the optical unit 12 is a color-combining element. The optical unit 12includes the light sources 1 a, 1 b, and 1 c that oscillate in the TEmode, and the color-combining element 3 that the light emitted from eachof the light sources 1 a, 1 b, and 1 c will enter as p-polarized lightand change a traveling direction thereof. In this configuration of theoptical unit 12, the major axis of a beam spot on a screen 6 can be madenearly perpendicular to a scanning direction, and high-resolution imagesfree from conspicuous optical-axis misalignment can be obtained.

FIG. 6 schematically shows a configuration of an optical unit accordingto a sixth embodiment of the present invention. The combining element 3in the optical unit 12 is a color-combining element. The optical unit 12includes the light sources 1 a, 1 b, and 1 c that oscillate in the TMmode, and the color-combining element 3 that the light emitted from eachof the light sources 1 a, 1 b, and 1 c will enter as p-polarized lightand change a traveling direction thereof. A plane where thecolor-combining element 3 is disposed differs from that where thescanning element 4 is disposed. In this configuration of the opticalunit 12, the major axis of a beam spot on a screen 6 can be made nearlyperpendicular to a scanning direction, and high-resolution images freefrom conspicuous optical-axis misalignment can be obtained.

FIG. 7A shows a light source 1 and optical element 2 of the optical unit12 of the present invention, and a screen 6. FIG. 7B shows the screen 6onto which an image is projected by the image display device using theoptical unit 12 of the present invention, and pixels in the scanningdirection on the screen. FIG. 7C shows one pixel on the screen 6 and onebeam spot formed thereon during projection by the image display deviceusing the optical unit 12 of the present invention.

The optical unit 12 set forth in claim 1 is configured to satisfy arelational expression of

B=2f tan θ+a≧Sscan/nscan≧φ

where a horizontal spot size of a beam on an exit surface of an opticalelement 2 is defined as B (mm); a size of the beam spot in a scanningdirection on a screen 6, as φ (mm); a distance from a light-emittingpoint of the light source 1 to the optical element 2, as “f” (mm); anangle formed by a spread direction of the light emitted from the lightsource with the optical axis, as θ(°); a size of the light-emittingpoint in a direction parallel to the scanning direction, as “a” (mm); asize of the screen 6 in the scanning direction, as Sscan (mm); and thenumber of pixels in a scan line in the scanning direction, as “nscan”.

Thus, the size of one pixel and that of the beam spot become the same,such that a high-resolution sharp image can be obtained.

In addition, since it suffices for the optical unit to satisfy the aboveexpression in the minor-axis direction of the spot, the presentinvention creates margins on lens design, compared with the case wherethe expression is applied to be satisfied in the major-axis direction,and lenses of small refractive power can be used. This makes the opticalunit manufacturable at a lower cost. Furthermore, the shape of thelight-emitting point can be rectangular as shown in FIG. 7A, or can besquare, round, or elliptical. Substantially the same effects can beobtained for practically all shapes.

FIG. 8 shows one pixel on the screen 6 and one beam spot formed thereonduring projection by the image display device using the optical unit 12of the present invention. The optical unit 12 in this case satisfies arelational expression of

B=2f tan θ+a≧1.5Sscan/nscan≧φ

where a horizontal spot size of the beam on the exit surface of theoptical element 2 is defined as B (mm); a size of the beam spot in thescanning direction on the screen 6, as φ (mm); a distance from thelight-emitting point of the light source 1 to the optical element 2, as“f” (mm); an angle formed by a spread direction of the light emittedfrom the light source with the optical axis, as θ (°); a size of thelight-emitting point in the direction parallel to the scanningdirection, as “a” (mm); a size of the screen 6 in the scanningdirection, as Sscan (mm); and the number of pixels in a scan line in thescanning direction, as “nscan”. That is, a high-resolution image can beobtained when the beam spot size on the screen 6 is up to 1.5 times asgreat as the pixel size.

FIG. 9 shows a beam spot formed on the screen 6 during projection by theimage display device using the optical unit 12 of the present invention.The optical unit 12 set forth in claim 1 is configured to satisfy arelational expression of

B=2f tan θ+a≧Sscan/nscan≧φ sin γ, or

B=2f tan θ+a≧1.5Sscan/nscan≧φ sin γ

where a horizontal spot size of the beam on the exit surface of theoptical element 2 is defined as B (mm); a size of the beam spot in thescanning direction on the screen 6, as φ (mm); an angle formed betweenthe scanning direction and the major axis of the spot, as γ (°); adistance from the light-emitting point of the light source 1 to theoptical element 2, as “f” (mm); an angle formed by a spread direction ofthe light emitted from the light source with the optical axis, as θ (°);a size of the light-emitting point in the direction parallel to thescanning direction, as “a” (mm); a size of the screen 6 in the scanningdirection, as Sscan (mm); and the number of pixels in a scan line in thescanning direction, as “nscan”.

That is, a high-resolution image can be obtained, even if the beam spothas the inclination angle γ with respect to the scanning direction. Theabove assumes that the inclination angle γ satisfies a requirement thatthe beam spot, when compared in size, is narrowed down in the scanningdirection than in a direction perpendicular thereto.

FIG. 10A shows red (R), green (G), and blue (B) beam spots formed on thescreen 6 during projection by the image display device using the opticalunit 12 of the present invention. The optical unit 12 that includes R,G, and B three-color light sources 1 a, 1 b, and 1 c, a plurality ofoptical elements 2 a, 2 b, and 2 c for controlling a spread of light, acolor-combining element 3 for combining the beams of light emitted fromthe light sources 1 a, 1 b, and 1 c, and a scanning element 4, satisfiesa relational expression of

φG≦1.5d, φRB<0.5d, φRG<0.5d, φBG<0.5d,

where a pixel pitch on the screen 6 is defined as “d”; a beam spot sizeof the G-light, as θG; a beam spot clearance between the R-light and theB-light, as φB; a beam spot clearance between the R-light and theG-light, as φRG; and a beam spot clearance between the B-light and theG-light, as φG.

For example, even if the positions or inclinations of the light sources1 a, 1 b, and 1 c or optical elements 2 a, 2 b, and 2 c contain anerror, when the above conditions are satisfied, the three colors,namely, R, G, and B, do not appear to be divided on the screen 6.Instead, the three colors are synthesized into white, with the resultthat a high-resolution image can be obtained. Conversely if either φRB,φRG, or φBG exceeds 0.5d, the colors appear to be divided on the screen6, the result being that resolution deteriorates.

FIG. 11 schematically shows a configuration of an optical unit accordingto a seventh embodiment of the present invention. In the optical unitset forth in claim 10, of all kinds of light from the light sources 1 a,1 b, and 1 c, only the R-light and the B-light enter the color-combiningelement 3 in the form of s-polarized light, and the G-light enters asp-polarized light. For example, when a dichroic prism or a dichroicmirror is used as the color-combining element 3, even when an emissiondistribution of the G-light is close to a broad one, high transmittancecan be obtained in a wide wavelength range by leading the G-light asp-polarized light to the color-combining element 3. High luminance cantherefore be achieved at a low cost with a minimum number of layerssmall in dichroic surface area.

FIG. 12 schematically shows a configuration of an optical unit accordingto an eighth embodiment of the present invention. In the optical unit 12of the eighth embodiment, the number of optical elements 2 a, 2 b, and 2c through which B-light is passed is the smallest in all otherembodiments described herein. The optical elements 2 a, 2 b, and 2 cusually decrease in transmittance and reflectance in a wavelength regionof 400 to 485 nm of the B-light. Reducing the number of optical elements2 a, 2 b, and 2 c through which the B-light is passed prevents thebrightness thereof from decreasing, and correspondingly improves theoptical unit 12 in luminance.

FIG. 13A schematically shows an example of time-varying changes inoptical output level that are observed when the optical unit of thepresent invention is driven. While the conventional driving methodemploys PWM driving based on fixed optical output (see FIG. 13B) oranalog driving based on non-pulse modulation (see FIG. 13C), the opticalunit 12 conducts PWM driving at variable optical output levels andrepeats integration of these output levels over a definite time, beforecalculating optical outputs of R, G, B.

If the time-varying changes in the optical output levels of thePWM-driven light sources are expressed as P(t), a color temperature Kwcalculated from chromaticity coordinates (xW, yW) derived from thefollowing expression will satisfy 10,000 K≦Kw≦16,000 K:

$\begin{matrix}{{X_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{X}(\lambda)}{\lambda}}}},{Y_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{Y}(\lambda)}{\lambda}}}},{Z_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{Z}(\lambda)}{\lambda}}}},{x_{W} = \frac{X_{R} + X_{G} + X_{B}}{\begin{matrix}{\left( {X_{R} + Y_{R} + Z_{R}} \right) + \left( {X_{G} + Y_{G} + Z_{G}} \right) +} \\\left( {X_{B} + Y_{B} + Z_{B}} \right)\end{matrix}}},{y_{W} = \frac{Y_{R} + Y_{G} + Y_{B}}{\begin{matrix}{\left( {X_{R} + Y_{R} + Z_{R}} \right) + \left( {X_{G} + Y_{G} + Z_{G}} \right) +} \\\left( {X_{B} + Y_{B} + Z_{B}} \right)\end{matrix}}}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where

∫_(t = 0)^(t)P(t)t

signifies a time integral of the optical output levels of R, G, B.

Image quality high in color temperature can therefore be obtained. Colortemperature is univocally determined from chromaticity coordinates(bibliography: http://www.spstj.org/book/pdf/lecture/lec_(—)2006.pdf).For example, a color temperature of 10,914 K and a deviation of 0.02 arederived from chromaticity coordinates of (xW, yW)=(0.26, 0.31).

FIG. 14 schematically shows a configuration of an optical unit accordingto a ninth embodiment of the present invention. In the optical unit 12of the ninth embodiment, the light sources 1 a, 1 b, and 1 c, theoptical elements 2 a, 2 b, and 2 c, and the color-combining element 3are each retained by an independent hold member, with a heat-releasingmember 11 being disposed in each hold member. Since the hold member andthe heat-releasing member 11 are both constructed independently, heatfrom the light sources 1 a, 1 b, and 1 c can be released without beingconducted to the optical elements 2 a, 2 b, and 2 c, the color-combiningelement 3, or the housing 444. This, in turn, prevents an optical outputloss in the light sources 1 a, 1 b, and 1 c, shifts in the positions ofthe respective light-emitting points, and peak wavelength drifts fromoccurring, and high image quality can be achieved as a result.

An embodiment of an image display device in which an electric powersupply unit 14, a circuit block 15, and a signal-processing unit arecombined in the optical unit 12 of the above configuration is shown inFIG. 15. According to this embodiment, a compact and lightweightimage-display device can be supplied.

1. An optical unit comprising: a light source that emits light to beprojected onto a screen; one or a plurality of optical elements thatcontrols a spread of a beam of light from the light source to thescreen; a combining element that combines the light emitted from thelight source; and a scanning element; wherein the light source outputsan optical beam that will generate an elliptically shaped beam spot onthe screen, the beam spot having a major axis substantiallyperpendicular to a scanning direction; and wherein the light source isdriven with pulse width modulation (PWM) so that one driven pulsecorresponds to one beam spot on the screen.
 2. The optical unitaccording to claim 1, wherein one pixel on the screen is depicted by aplurality of driven pulses.
 3. The optical unit according to claim 1,wherein the optical output levels of each driven pulses are varied. 4.The optical unit according to claim 1, wherein the light source includesa three-color light sources of red (R), green (G), and blue (B), and iftime-varying changes in optical output levels of the light source asdriven with pulse width modulation (PWM) are each expressed as P(t), acolor temperature Kw that is calculated from chromaticity coordinates(xW, yW) derived from an expression of${X_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{X}(\lambda)}{\lambda}}}},{Y_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{Y}(\lambda)}{\lambda}}}},{Z_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{Z}(\lambda)}{\lambda}}}},{x_{W} = \frac{X_{R} + X_{G} + X_{B}}{\begin{matrix}{\left( {X_{R} + Y_{R} + Z_{R}} \right) + \left( {X_{G} + Y_{G} + Z_{G}} \right) +} \\\left( {X_{B} + Y_{B} + Z_{B}} \right)\end{matrix}}},{y_{W} = \frac{Y_{R} + Y_{G} + Y_{B}}{\begin{matrix}{\left( {X_{R} + Y_{R} + Z_{R}} \right) + \left( {X_{G} + Y_{G} + Z_{G}} \right) +} \\\left( {X_{B} + Y_{B} + Z_{B}} \right)\end{matrix}}}$ satisfies 10,000 K≦Kw≦16,000 K.
 5. The optical unitaccording to claim 1, wherein the light source, the optical elements,and the combining element are each retained by an independent holdmember, and a heat-releasing member is disposed in each of the holdmembers.
 6. An optical unit comprising: three, light sources arranged toemit red (R), green (G), and blue (B) light respectively andindependently, suitable for projection onto a screen, the light sourcesconfigured to provide time-varying changes in optical output; aplurality of optical elements arranged to control a speed of light; acolor-combining element arranged to combine the color of the lightemitted from the light sources; and a scanning element configured toscan the color-combined light across a viewing plane across a number ofpixels, each pixel having a pixel pitch in a scanning direction, thepixel pitch being determined by a scanning distance over which the lightsources are activated; wherein the light sources are arranged to outputan optical beam that will generate an elliptically shaped beam spot inthe viewing plane, in which the screen is located in use, the beam spothaving a major axis substantially perpendicular to a scanning direction;and wherein the optical unit is configured to control the spread oflight in the viewing plane to the relational expression of1.0d≦φG≦1.5d, 1.0d≦φR≦1.5d, 1.0d≦1.5d, φRB<0.5d, φRG<0.5d, φBG<0.5d,where a pixel pitch at the viewing plane is defined as “d”; a beam spotsize at the viewing plane of the G-light, as φG; and a beam spot size atthe viewing plane of the R-light, as φR; a beam spot size at the viewingplane of the B-light, as φB, a beam spot clearance between the R-lightand the B-light, as φRB; a beam spot clearance between the R-light andthe G-light, as φRG; and a beam spot clearance between the B-light andthe G-light, as φG, the beam spot size and the beam spot clearance beingtaken in a direction perpendicular to the scanning direction.
 7. Theoptical unit according to claim 6, wherein the color-combining elementis formed so that of all components of the light from the light sources,only the R-light and the B-light enter that in an s-polarized state andthe G-light enters that in a p-polarized state.
 8. The optical unitaccording to claim 6, wherein a of the optical elements through whichthe B-light passes is smaller than that through which optical beams ofthe other colors pass.
 9. The optical unit according to claim 6,wherein, if time-varying changes in optical output levels of the lightsource as driven with pulse width modulation (PWM) are each expressed asP(t), a color temperature Kw that is calculated from chromaticitycoordinates (xW, yW) derived from an expression of${X_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{X}(\lambda)}{\lambda}}}},{Y_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{Y}(\lambda)}{\lambda}}}},{Z_{B} = {\int{\left( {\int_{t = 0}^{t}{{P(t)}{t}}} \right) \times {\exp \left( {{- x^{2}}/2} \right)} \times {\overset{\_}{Z}(\lambda)}{\lambda}}}},{x_{W} = \frac{X_{R} + X_{G} + X_{B}}{\begin{matrix}{\left( {X_{R} + Y_{R} + Z_{R}} \right) + \left( {X_{G} + Y_{G} + Z_{G}} \right) +} \\\left( {X_{B} + Y_{B} + Z_{B}} \right)\end{matrix}}},{y_{W} = \frac{Y_{R} + Y_{G} + Y_{B}}{\begin{matrix}{\left( {X_{R} + Y_{R} + Z_{R}} \right) + \left( {X_{G} + Y_{G} + Z_{G}} \right) +} \\\left( {X_{B} + Y_{B} + Z_{B}} \right)\end{matrix}}}$ satisfies 10,000 K≦Kw≦16,000 K.
 10. The optical unitaccording to claim 6, wherein the light sources, the optical elements,and the color-combining element are each related by an independent holdmember, and a heat-releasing member is disposed in each of the holdmembers.